In recent years, there is rapid progress of conductive circuits with a high density. The conventional subtractive process used for forming the conductive circuits in which a copper foil laminated on an insulating substrate is etched for patterning thereof requires a prolonged time and is complicated, resulting in production of a large amount of wastes. In consequence, instead of the subtractive process, a printing process or a coating process using a conductive paste comprising conductive particles to form the conductive circuits has been noticed. For example, in a screen printing method generally used for circuit printing, flake-like metal particles having a particle diameter of not less than several micrometers or the like are used as the conductive particles to form a circuit having a thickness of not less than 10 μm and thereby ensure a conductivity thereof. In order to form a circuit having a higher density, still finer metal particles have been developed.
Hitherto, as a conductive material formed on circuit boards, there has been used a paste obtained by using metal particles such as particles of copper, silver, a copper alloy, gold, platinum, silver-palladium, etc. Among these metals, in view of conductivity and stability with time, silver, gold, platinum and silver-palladium, in particular, silver, are generally used as the conductive particles. However, gold, platinum and silver-palladium are not only expensive by themselves but also have such a problem that these sources will be exhausted in near future, and silver further has the problem concerning ion migration generated between circuits under high-temperature and high-humidity conditions.
In consequence, it has been attempted that copper as a more inexpensive and rich source and a copper alloy are used as alternative conductive metal particles in place of silver. For example, metal fine particles having a particle size of not more than 100 nm are extremely active unlike ordinary metal particles having a particle size of not less than several micrometers and therefore undergoes a phenomenon of lowering of melting point. With respect to copper, study has also been made on low-temperature calcination utilizing the phenomenon of lowering of melting point.
However, since copper particles tend to readily form an oxide layer on a surface thereof, there tends to arise such a problem that the copper particles are deteriorated in conductivity owing to the oxide layer. In addition, as the particle size of the copper particles is reduced, the adverse influence of the oxide layer on a conductivity of the particles tends to become more remarkable. In consequence, in order to reduce the oxide layer on the copper particles, it is required that the copper particles are subjected to reducing treatment at a temperature exceeding 300° C. in a reducing atmosphere such as hydrogen or to sintering treatment at a much higher temperature. At this time, the conductivity of the copper particles becomes closer to that of a bulk copper. However, even the thus treated copper particles can be used only in limited applications in which an insulating substrate used therewith must be formed of a high heat-resistant material such as ceramic materials and glass.
As the method of forming a conductive coating film, there is known a plating method. Further, as the plating method, there are known a full additive process in which only a wiring portion of an insulating substrate is subjected to electroless plating, a semi-additive process using resist printing in combination with electroplating, and a panel plating process in which an insulating substrate having through-holes is successively subjected to electroless plating and then copper electroplating to form a copper plating layer thereon and thereafter etch the copper plating layer.
In the conventional arts, there is known the method in which a undercoat layer for electroless plating is provided on an insulating substrate, and then the insulating substrate is subjected to electroless plating to form an electroless plating layer on the undercoat layer. In Patent Document 1, it is described that a composite metal oxide hydrate comprising Pd or Ag and a binder are used as raw materials for forming a undercoat layer for electroless plating. In Patent Document 2, it is described that a surface of a pattern formed from a metal paste is subjected to sensitization treatment and then to electroless plating. In Patent Document 3, it is described that only a circuit portion having a specific resistance value of not more than 10·cm which is formed of a conductive paste is subjected to electroless plating.
However, in the above conventional arts, the electroless plating process requires an activation treatment using palladium in order to enhance a deposition ability or a selectivity for the plating. Palladium is not only expensive, but also requires a complicated process maintenance including previous or subsequent steps for the activation treatment.
In addition, in the electroless copper plating process used for forming circuits, formaldehyde is generally used as a reducing agent. In general, such a plating process using formaldehyde has been conducted in the presence of a strong alkali and under high-temperature conditions in order to enhance an activity of the formaldehyde. As a result, there tends to occur deterioration in adhesion between the plating layer and the insulating substrate as well as occurrence of defects on the plating layer. These drawbacks are one of causes inhibiting spread of such a circuit forming method in which after printing a circuit using a copper paste, a conductive circuit is formed thereon by electroless copper plating.
In addition, in the above conventional arts, there are known no methods in which an electroless plating layer is formed on a metal paste coating film without adding a noble metal such as Ag to the paste and without conducting an activation treatment using palladium or a sensitization treatment. Therefore, characteristics of copper particles for forming a electroless plating layer without conducting these treatments have been not apparently recognized.
Also, conventionally, for the purpose of preventing oxidation of copper fine particles, there has been attempted the method of treating the surface of the particles with benzotriazole or derivatives thereof, an organic amine, a fatty acid, a metal alkoxide, etc. In Patent Document 4, there are described copper fine particles treated with a fatty acid. However, it is not possible to produce a high-quality electroless plating layer on a coating film formed of a paste using the copper particles having such a surface treatment layer.
In addition, in Patent Document 5, there are described copper fine particles having a particle diameter of 20 to 41 nm which are produced by a liquid phase method. In Patent Document 6, there is described the method for producing metal fine particles from a metal compound in a liquid phase using a polyol as a reducing agent.
Also, in Patent Documents 5 and 7, it is described that a copper paste coating film is subjected to heat treatment in a nitrogen atmosphere at a temperature of 300 to 350° C. for 1 hr to thereby obtain a thin film having an excellent conductivity. However, the fine particles have a very large surface area and therefore are likely to be extremely readily oxidized, so that there tend to arise the problems such as deterioration in conductivity of the particles and increase in temperature required upon calcination thereof. As a result, it may be difficult to form a conductive coating film having a low volume resistivity on a substrate formed of a material having a relatively low heat resistance such as organic materials. In particular, the copper fine particles tend to suffer from defects owing to oxidation thereof, and therefore tend to be deteriorated in conductivity.
In addition, in Patent Document 8, it is described that in order to reduce an oxide layer of purified copper particles, a cupric oxide dispersion comprising ethylene glycol is treated with a hydrogen gas at 250° C. to obtain a copper thin film having an excellent conductivity. However, the method fails to provide a suitable method having a high productivity because the high-temperature treatment is conducted in the highly explosive hydrogen gas.
In consequence, for the purpose of preventing oxidation of copper fine particles, there has been attempted the method of treating the surface of the particles with benzotriazole or derivatives thereof, an organic amine, a fatty acid, a metal alkoxide, etc. In Patent Document 4, there are described copper particles treated with a fatty acid. However, when removing such a surface treatment layer, it is also required to conduct a heat treatment in a high-temperature atmosphere, so that it may be difficult to obtain a conductive coating film on a substrate formed of a resin film, etc.
Also, in Patent Document 9, there has been proposed the method of enhancing a conductivity of a coating film formed of a dispersion of metal fine particles by subjecting the coating film to heat treatment with superheated steam. However, the characteristics of copper particles which exhibit a good conductivity when subjected to the superheated steam treatment have been not clearly determined therein.
For example, in the case where copper particles are subjected to surface treatments for the purpose of suppressing oxidation of the copper particles or preventing increase in viscosity of a paste upon preparation thereof, the activity on the surface of the copper particles can be suppressed, so that a storage property of the copper particles and a stability of the paste can be enhanced. However, the effects of the heat treatment with superheated steam also tend to be suppressed by the surface treatments, so that it is not possible to obtain a conductive coating film having an excellent conductivity.